Face milling cutters with indexable cutting inserts have been known for a number of years and have come into widespread use because they minimize expensive downtime. An indexable cutting insert has a number of identical and alternatively usable straight cutting edges, so that when an operative one of those edges becomes dulled by use the insert can be indexed--removed from the cutter body and reinstalled therein in a different orientation--to bring a new one of those edges to the operative position.
An indexable cutting insert comprises a small block of metal having a pair of opposite larger surfaces that are flat and parallel to one another and having a plurality of smaller flat surfaces, one for each of the edges on the insert, each edge being defined by the intersection of a smaller flat surface with one of the larger ones. (Circular indexable cutting inserts, employed for certain special purposes, are not of interest in connection with the present invention.) By way of example, an indexable insert for cutting with positive rake may be triangular to have three edges, square to have four edges, or octagonal to have eight edges. In connection with the present invention the only insert geometries that are of interest are those that provide either four or eight straight edges, that is, inserts that are essentially square and octagonal.
In operation, a face milling cutter typically rotates on a fixed vertical axis and cuts into a workpiece near a top surface thereof as the workpiece is steadily fed in one horizontal direction in engagement with the cutter. In effect, the cutter somewhat reduces the height of the workpiece to provide it with a finished top surface area that is flat and lies in a plane exactly normal to the cutter axis. As the workpiece is fed in engagement with the cutter, cutting is effected by the peripheral edges on the inserts, which thus produce and successively cut into an arcuate shoulder that extends more or less vertically between the finished top surface and the unfinished one. The angle between this shoulder and the finished flat surface corresponds to the lead angle of the peripheral cutting edges of the inserts. Lead angle, sometimes referred to as the cut-entering angle, is the acute angle between the peripheral cutting edge on the insert and a parallel to the cutter axis which intersects that edge.
An indexable cutter insert is usually configured for one of a number of more or less standardized lead angles, namely: 45.degree., 30.degree., 15.degree. and substantially 0.degree., the last mentioned including lead angles on the order of 1/2.degree. to 3.degree., which are usually close enough to 0.degree. for practical purposes. Apart from requirements that may be imposed by the need for a particular angle for the arcuate shoulder on the workpiece, the selection of lead angle is important because of its bearing on production economy. Other things being equal, the rate of feed of the workpiece can be increased with increasingly large lead angles; but the larger the lead angle, the smaller the possible depth of cut. Thus it is necessary to provide cutters with a full range of lead angles in order to accommodate all of the conditions and requirements likely to be encountered in face milling a variety of workpieces.
The present invention, which provides a face milling cutter having significant economic and functional advantages over those heretofore available, rejects certain heretofore accepted teachings in the face milling art. One of these has to do with a supposed necessary relationship between the cutter body and the lead angle of its inserts.
The following appears in .sctn.74 of "Milling with Indexable Inserts/Programmed Instruction", published by Fansteel-VR/Wesson (no date): "Body selection narrows the range of inserts that can be used. For example, the geometry of the body will determine the lead angle of the insert." Similarly, in "Milling Handbook of High-Efficiency Metal Cutting", copyrighted in 1980 by General Electric Company, the following appears at p. 17: "Selection of the right cutter body can often mean the difference between success and failure in a milling operation. The following are the factors which must be considered: . . . (5) Lead angle . . . "
Provision of a different cutter body for each different lead angle involved a costly inventory problem for a manufacturer of face milling cutter bodies who wanted to provide prompt delivery service. A given cutter body can have only one diameter, is specialized for either right-hand or left-hand cutting rotation, and can have only one combination of radial and axial rake angles. Thus, stocking a complete line of cutter bodies required at least one body for each of the several lead angles and their combinations and permutations with diameter, rotation direction and radial and axial rake angles. Bearing in mind that the cost of a cutter body is almost never less than $200, and is often several hundred dollars, the investment represented by such an inventory is very substantial. Reducing this variety to a small fraction of what it has heretofore been, by enabling a given cutter body to accommodate all of the standard lead angles, is very advantageous to users of face milling cutters as well as to manufacturers of them. Evidently it has not heretofore been obvious how this can be achieved.
Another of the heretofore accepted teachings that has been rejected for the purposes of the present invention relates to the radial and axial rake angles of the cutting inserts. As stated (in solid capital letters for strong emphasis) at p. 24 of the above mentioned General Electric publication: "Select double-negative cutters whenever the workpiece and machine tool will allow, unless positive tooling offers an economic advantage." The face milling cutter of the present invention uses positive radial rake and positive axial rake for all materials and milling conditions, thereby obtaining the economic and functional advantages heretofore recognized as available with double-positive rake--lower cutting forces and lower power consumption--but also achieving further important advantages that had not heretofore been expected, including reduction of cutter body inventory requirements, faster milling with smoother surface finishes, and longer useful life of the inserts between sharpenings.
For the body of the cuter of this invention certain conventional wisdom concerning the geometry of face milling cutter bodies is deliberately disregarded, and specifically the accepted teachings concerning cutter bodies for lead angles substantially larger than 0.degree.. Heretofore the body of a cutter for a lead angle of 15.degree., 30.degree. or 45.degree. has usually had an overall diameter substantially greater than the effective diameter of the cutter. Such cutters were therefore not well suited for automatic tool changer installations, for which both the weight and the bulk of a cutting tool should be as small as possible. The cutter of the present invention has an unprecedented adaptability for use with automatic tool changers because--for every lead angle--its maximum body diameter is smaller than its effective diameter. As will appear from the following description, this favorable body geometry is one of the advantages resulting from the unconventional use of double-positive rake for all materials and milling conditions.
The cost of a set of cutting inserts for a face milling cutter is only a fraction of the cost of the body, and indexable inserts, because of their multiple alternatively usable cutting edges, provide especially favorable economy in terms of tool cost per cut. In fact, users of indexable inserts were heretofore given some encouragement to discard them when all of their cutting edges had been dulled by use, rather than having them resharpened. More recently the materials used in carbide milling cutter inserts have become relatively expensive, so that it is now notably wasteful to discard carbide inserts that are capable of being resharpened.
However, for computer-controlled milling machines, resharpening of face milling cutter inserts has heretofore presented an additional cost that has been significant. When an insert is resharpened, its size is somewhat reduced by reason of the removal of metal from it. If the insert is installed in a body that has a fixed surface which establishes its axial position relative to the body, resharpening effectively raises the operative cutting edge on the insert. If such a cutter is brought to a given axial position during a computer-controlled face milling operation, the cut that it makes with resharpened inserts is not quite as deep as the one it made with the same inserts before resharpening. It is not practical to reprogram the machine after each resharpening of inserts, and therefore cutter bodies intended for use with automatically controlled machines were usually arranged to require individual axial adjustment of the inserts as they were installed, to accommodate reduction in insert size due to resharpening. This was a tedious and time consuming process, requiring that each insert be lightly clamped in the body, then tapped with a mallet to shift it back and forth in the body, checking its position with a gauge after each such shift, until it was finally brought to exactly the desired position, whereupon it was clamped tight. Such adjustment of a set of inserts usually required several hours and thus involved costly labor time as well as unavailability of the cutter.
As pointed out above, the milling cutter body of this invention can be adapted for cutting at any selected lead angle by simply installing a different set of inserts in it whenever the lead angle is to be changed. Obviously, some of the advantages of this versatility would be sacrificed if every change of inserts entailed the tedious and laborious adjustment procedure just described. Thus the versatility contemplated by this invention requires the cutter body to be so configured and arranged as to provide for quick and easy installation of each of the several different inserts intended for cooperation with it, whereby each of the operative edges of every such insert is accurately disposed in a predetermined position and orientation. Furthermore, the body must provide for such facile installation of resharpened inserts as well as of those that are new, without need for tedious or difficult adjustment to compensate for the reduction in insert dimensions that results from resharpening.
Three prior U.S. patents are known to the applicant which hindsight can endow with a semblance of pertinence but which did not in fact offer a solution to the complex of problems addressed by this invention: Markstrum, U.S. Pat. No. 1,927,409 (1933); Connell, U.S. Pat. No. 2,351,491 (1944); and Begle et al, U.S. Pat. No. 2,690,610 (1954).
The cutting inserts disclosed by Markstrum are removable from the cutter body but are not indexable. Each insert is elongated to have a front end portion on which its cutting edge is formed, and it is installed in the body with its length at a marked forwardly and radially outwardly oblique inclination to the cutter axis. The cutter body has a rearwardly projecting, reduced diameter neck portion onto which a nut-like collar is threaded. The collar has a flat front surface that engages a rear end of each insert, so that rotation of the threaded collar to shift it forwardly along the neck effects a corresponding forward shift of all of the inserts. Because the inserts are lengthwise oblique to the cutter axis, their forward axial displacement also involves a radial component that increases the radius of their cutting edge orbits; but the ratio of radial to axial shifting may vary from insert to insert because of the way the inserts are mounted in the body. Each insert is received in a groove in the body that is radially outwardly divergent, and the insert tapers in thickness to have a wedging fit in the groove. Unless the groove surfaces and their mating surfaces on the inserts are machined with an almost unattainable precision, the cutting edges on the several inserts will be at different distances from the cutter axis, and because of this radial run-out the tool would have very poor cutting action. Notwithstanding such pertinence as it might seem to have in the light of the present invention, the Markstrum patent did not in fact teach the art how to overcome the problems solved by this invention, both because the cutter of that patent did not have indexable inserts and because it presented other and more troublesome problems that were not encountered by heretofore conventional cutters with indexable inserts.
Connell discloses two inserted tooth cutter embodiments, one a reamer, the other a milling cutter. Neither has indexable inserts. In the reamer, each insert-receiving slot in the body defines a flat, radially outwardly facing locating surface that is oblique to the body axis, being inclined forwardly and radially outwardly to it; and each of the inserts tapers in radial thickness along its length and engages this locating surface to be displaced radially outwardly in consequence of being shifted forwardly along it. As with the Markstrum device, the Connell reamer body has a rearwardly projecting neck onto which is threaded a collar that engages rear end portions of the inserts to provide for their simultaneous axial shifting. The milling cutter disclosed by Connell does not include the threaded adjusting collar that Connell provides for the reamer. Instead, the means for effecting axial positioning of the inserts requires each insert to be adjusted individually and involves a slot in each insert that would be incompatible with any practical indexable insert geometry.
Begle et al discloses two embodiments of indexable insert milling cutters, but both must be arranged for negative axial rake and negative radial rake and they are thus unsuitable for many industrially important nonferrous metals such as brass and aluminum. In the more nearly pertinent of the two embodiments disclosed by Begle et al, each cutting insert is an octagonal block having opposite and parallel flat faces and providing a total of 16 cutting edges. These octagonal inserts, which provide a 45.degree. lead angle, closely correspond to present-day indexable inserts. The cutter body in which they are installed has fixed circumferential, radial and axial locating surfaces. In this connection the patent says: "It is contemplated that the cutting blades 52 be thrown away or discarded after all the sixteen cutting edges have been used; and if this is done, the necessity of grinding or adjusting the blades is entirely eliminated." As this statement implies, and as is apparent from the patent disclosure, the cutter body has no means for compensating for the reduced size of resharpened inserts, nor is the cutter suitable for other than a 45.degree. lead angle.
The other milling cutter embodiment disclosed by Begle et al includes provision for individual axial adjustment of the inserts to accommodate resharpening, but in this case the inserts, although indexable, are substantially elongated and are thus materially different in shape from now-conventional indexable inserts. The cutter body that accommodates such inserts has great axial length and therefore has weight and bulk that are unsuitable for automatic tool changers.